Bring water and quinoa to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is tender and water has been absorbed, 15 to 20 minutes. Cool quinoa completely.
Mix crabmeat, cucumber, tomatoes, and feta cheese together in a large bowl; add quinoa and stir. Whisk Greek salad dressing, lemon juice, and balsamic vinegar together in a separate bowl; drizzle over the quinoa mixture and stir to coat.